Re: Classic reponse from Mozilla
Thunderbird is probably the most comprehensive and shiny GUI mail client available on Linux, but it is also horribly bloated.
Claws mail is the best alternative as far as I can see (and not vulnerable to this attack IIRC) - it's GUI enough to not need years of fiddling to become comfortable with, feature-complete for almost everyone, reasonably bloat free and dead stable.
Not brilliant for viewing HTML email, but then again I'd rather not have a full blown web browser embedded in my email client anyway...
(The situation isn't any better on Windows by the way, proper email clients are sadly not fashionable and nobody seems interested in working on them.)